The present invention relates in general to a system for the measurement of the difference between two fluid flows in two separate ducts using one or more monitoring electrodes or the like, arranged in respective ducts, and more particularly, to the measurement of relatively small differences between two relatively large fluid flows.
The present invention may also be applied, for example, to the measurement of ultrafiltration in a dialysis system, where the continuing clean dialysis solution flows through one duct, while the same solution, i.e., spent dialysis solution, but increased through ultrafiltration in the dialyser, flows through the other duct. The two main fluid flows may be in the order of magnitude of about 500 ml/min, whereas their difference, that is the ultrafiltration, may be in the range of about 0 to 40 ml/min and exceptionally a little higher.
In British Pat. No. 2 003 274 and in British Patent Application No. 2 056 691, two prior art systems for electromagnetic measurement of the differences between two fluid flows are described. In both systems, the fluid flows whose difference are to be measured pass through two parallel ducts. In accordance with the British patent, the measurement is performed using three electrodes which are acted upon as a function of the flowing medium and an externally applied magnetic field. In accordance with the British patent application, two main electrodes which cooperate with a number of earth electrodes are instead acted upon in the same manner.
Both prior art systems suffer from a number of notable disadvantages, namely that the flowing media acts upon the two ducts, or the electrodes, in a different manner. If, for example, such prior art systems are used for the measurement of ultrafiltration in dialysis, deposits from the contaminated liquid are often obtained on the electrodes and/or on the actual duct walls. Even a deposit in the order of magnitude of a few microns may cause substantial errors in the readings.